The invention relates to an electrically assisted, power steering system for motor vehicles with an input shaft, which is in active connection with a steering wheel for transfer of a steering torque, with an output member, which is in active connection with the wheels to be steered, with a servo motor, which exerts power assistance directly or indirectly to the input shaft or the output member, wherein the input shaft and the output member are connected to each other by a torsionally elastic member such that limited rotational motion is possible between the input shaft and the output member, and with a detection unit, which contains at least one sensor for detecting the direction and the intensity of the steering torque acting on the input shaft, wherein the input shaft is supported on its end facing the steering wheel in a housing by a rolling bearing.
Such electrically assisted power steering systems for motor vehicles are superior in many respects relative to hydraulic servo steering systems. They work with an electronically regulated electro-motor and thus replace the conventional hydraulic system. This reduces the technical expense, because for regulation and steering support, only an unproblematic medium, electrical current, is still necessary. Simultaneously, the manufacturer can increase its productivity thanks to simpler system integration, faster assembly, and also optimum adaptation to the vehicle. And because the electrical control is adjusted optimally to the riding dynamics and vehicle handling, the ride properties of the car are significantly improved. In addition, this new type of steering requires significantly less energy than the hydraulic solution, because energy is needed only when steering is actually performed. A difference from conventional servo steering, no permanent hydraulic pressure is required. Outside of the servo unit, there are no aggregate systems: steering valve, steering pump, oil reservoir, and high-pressure hoses can be eliminated. This saves weight and simplifies assembly.
Such advantageous, electrically assisted, power steering systems are sufficiently known to someone skilled in the art. These can be taken, e.g., from DE 197 47 638 C1, DE 198 28 513 A1, DE 199 35 283 A1, and DE 298 20 079 U1. The steering torque is measured by a detection unit, which consists of at least one pulse generator and one sensor. The pulse generator and sensor are arranged at a distance from each other, wherein one of the two parts is connected to the input shaft. The input shaft is held in turn by a rolling bearing in a housing. It is obvious that the evaluation of the steering torque is influenced by the rolling bearing. For example, if there is too much bearing play, the function of the signal transmitter or sensor can be influenced in a negative way. This then transmits false control values to the servo motor, which has a negative effect on the steering behavior.
According to the prior state of the art, attempts have been made to overcome these disadvantages by exposing the input shaft to an exact guidance. This exact shaft guidance should enable in turn precise functioning of the pulse generator and sensor, which has a positive effect on the steering behavior, because the correct dosing of power from the support motor is introduced according to the real, determined steering torque. This was accomplished by the use of a solid rolling bearing, which was processed for reducing its radial play in a very expensive manner. This was achieved by grinding the tracks of the bearing. However, such a complicated solid rolling bearing is very expensive and also influences the costs of the power steering system.